The present invention relates generally to a method of testing an analog-to-digital converter (hereinafter referred to also as A/D converter or analog/digital converter). More particularly, the invention concerns a method of testing an A/D converter of the successive comparison type, as well as a structure of the A/D converter which is suited for application of the testing method.
In general, an A/D converter of the successive comparison type incorporates therein a digital-to-analog converter (hereinafter referred to also as D/A converter or digital/analog converter), a voltage comparator and a register for successive comparison. The precision of conversion or the conversion accuracy of such A/D converter has been measured heretofore through an analog measuring method or digital measuring method. According to the analog measuring method, a steadily increasing input voltage is applied to the A/D converter under test to be converted into a digital signal which is then restored to an analog signal by means of a reference D/A converter exhibiting accurate conversion characteristics. The restored or regenerated analog voltage is compared with the input voltage, to detect a possible difference on the basis of which the conversion characteristic of the A/D converter under test is determined. On the other hand, in the case of the digital measurement test, a digital set value is converted into an analog voltage through a reference D/A converter, which analog voltage is again converted into a digital signal through the A/D converter under test. The digital signal thus obtained is then compared with the original digital set value for determining the conversion accuracy of the A/D converter under test. The reference D/A converter employed in these testing methods should of course exhibit a sufficiently high conversion accuracy as compared with that of the D/A converter to be tested, in order to attain a high accuracy in the resultant measurement, which in turn means that a lot of time is required for the measurement or test, which is a disadvantage.
For example, when a 10-bit A/D converter is to be tested by using a reference D/A converter of 14-bit length according to the hitherto known measuring or testing method, the measurements will have to be repeated 2.sup.14 (=16,384) times by varying the input level to the 10-bit A/D converter under test, in order to effect the measurement or test over the whole input range. In this manner, the measurement of the conversion accuracy of the A/D converter with a reasonably high precision or accuracy requires an intolerably long time, involving high expenditure of effort for the test, while the measurement accuracy may be statistically degraded due to various influences such as externally applied noise.
Further, it should be mentioned that the hitherto known testing method requires a voltage comparator in addition to external measuring devices such as, for example, a computer and the reference D/A converter.
As the other prior art, conventional analog or digital measuring methods are disclosed in "Testing Digital/Analog And Analog/Digital Converters", Jim R. Naylor, IEEE Transactions on Circuits and Systems, Vol. CAS-25, No. 7, July 1978.